Spark plug



FIG. 1 is a perspective view of a spark plug showing my new design;

FIG. 2 is a front elevational view thereof; the rear, right side and left side elevational views being identical thereto;

FIG. 3 is a side elevational view intermediate to the views of FIG. 2;

FIG. 4 is a top plan view thereof; and,

FIG. 5 is a bottom plan view thereof. 

I claim the ornamental design for a spark plug, as shown and described. 